As an inverter for electric vehicles, a power semiconductor module (IGBT module) which is capable of making a high-voltage, large-current operation is being used. And, in recent years, for power semiconductor modules, realization of high output power and high integration have rapidly being progressed, and ceramic circuit substrates have increasingly been demanded to have a sufficient strength against a thermal stress in bonding and a sufficient bonding strength between a ceramic substrate and a metal plate against thermal cycling in severe atmosphere. As the circuit substrate for use in power semiconductor modules, a ceramic circuit substrate with which a metal plate, such as a copper plate, an aluminum plate, or the like, is bonded to a ceramic substrate composed of aluminum nitride or silicon nitride is widely used. This ceramic circuit substrate is formed by, for example, bonding a copperplate for circuit to load a semiconductor chip, and the like, to one face of a ceramic substrate, while bonding a copper plate for heat dissipation on another face. The above-mentioned copperplate for circuit has a circuit pattern composed of a plurality of copper plates which, by generally providing an etching treatment, or the like, become circuit parts. Herein below, in order to clarify the problems of the prior art, the present invention will be explained by using a copper plate as the metal plate, however, the present invention is not limited only to a case where a copper plate is used as the metal plate.
As a means for bonding between a ceramic substrate and a copper plate, the following means is being used. In other words, the direct copper bonding method (DCB method: Direct Copper Bonding method) is available which utilizes an eutectic liquid phase, such as Cu—Cu2O, or the like, to directly bond a copper plate to a ceramic substrate. In addition, the high melting point metal metalizing method is available which bakes a high melting point metal, such as Mo or W, on a ceramic substrate to form a metallic layer. Further, the active metal method is available which places a metal plate on a ceramic substrate coated with a brazing material containing an active metal, such as a 4A family element or a 5A family element, heating it at a moderate temperature while applying a pressing force thereto, thereby bonding the copper plate to the ceramic substrate through a brazing material layer formed of the brazing material. Any of the ceramic circuit substrates which are obtained by the DBC method and the active metal method offer such advantages as that the structure is simple, having a low thermal resistance, and can cope with semiconductor chips of large-current type and high integration type.
In addition, as a means for forming a circuit pattern of a copperplate for circuit, the following means is being used. In other words, the direct loading method is available which bonds a copper plate for circuit on which a circuit pattern is previously formed by press method or etching method to a ceramic substrate through a brazing material layer. In addition, the multistage etching method is available which forms a brazing material layer on substantially the entire face of a ceramic substrate, and bonds a copperplate so as to cover it, thereafter, making an etching treatment of both the copper plate and the brazing material layer to form a circuit pattern. Further, a method which uses brazing material pattern printing and etching in combination (hereinafter, referred to as the pattern printing etching method) is available which forms a brazing material layer along the geometry of a circuit pattern, and places a copper plate so as to cover this brazing material layer, then making an etching treatment of the copper plate in the same manner as with said multistage etching method for forming the circuit pattern.
Conventionally, of the above-mentioned means for bonding between ceramic substrate and metal plate, because a high strength, a high adhesion, and the like are obtained, the active metal method, using a brazing material paste in which an active metal, such as Ti, is added to an eutectic brazing material having an eutectic composition of Ag and Cu (72 mass % Ag-28 mass % Cu), is generally used. However, in the case where a brazing material having an Ag—Cu based eutectic composition as mentioned above is used for bonding a copper plate to a ceramic substrate, the brazing temperature is increased, because the melting point of the brazing material is high. If a ceramic substrate is bonded to a copper plate at a high brazing temperature, a residual stress attributable to the difference in coefficient of thermal expansion between both caused a warpage in the ceramic circuit substrate in some cases.
Further, with a ceramic circuit substrate on which a power semiconductor module is mounted, the thermal cycle applied at the time of starting or stopping the power semiconductor module causes a thermal stress to be repetitively imposed, being superimposed on the above-mentioned residual stress. And, if the resultant force of the residual stress and the thermal stress becomes too high to be withstood, the copper plate was peeled off from the ceramic substrate in some case. From such problems presented at the time of bonding between a ceramic substrate and a copper plate, and at the time of using a ceramic circuit substrate, a brazing material which is low in melting point has been demanded, and one example thereof has been proposed in the following Patent Document 1.
In Patent Document 1, in order to provide a brazing material which has a high bonding strength, being stable, and low in melting point, there is a disclosure of a paste brazing material which compounds a mixture of a powder mixture of an Ag—Cu—In alloy powder and a Ti powder, an organic solvent, and a resin, the brazing material preferably having a specific composition of 30 to 60% of Ag, 20 to 45% of Cu, 20 to 40% of In, and 0.5 to 5% of Ti.
As a result of examination of the brazing material disclosed in Patent Document 1 in which a large amount of In is added in order to lower the melting point of the brazing material, the present inventors have found that there is a problem that the bonding strength between the ceramic substrate and the copper plate is decreased. The cause of this is as follows. In other words, a brazing material in which a large amount of In is added has a low melting point, allowing the brazing temperature to be lowered. However, on the surface of a brazing material layer formed of such a brazing material, scale-like irreguralities are formed. And, it has been found that, resulting from the scale-like irreguralities, voids (vacancies) are generated at the bonding interface between the brazing material layer and the ceramic substrate, and the voids cause the bonding strength to be lowered. Herein, the convex part of the scale-like irregularities formed on the surface of the brazing material layer provides an Ag—In phase and a Cu—In phase which are formed like an island, while the concave part providing a Ti—Cu phase. With a brazing material in which a large amount of In is added, at the bonding interface between the brazing material layer and the ceramic substrate, a number of Ag—In phases and Cu—In phases, which constitute convex parts, and a number of Ti—Cu phases, constituting concave parts, are distributed, and it has been found that such concave parts give voids to thereby lower the bonding strength between the copper plate and the ceramic substrate.
Then, the present inventors have proposed a brazing material in which Ag powder particles having an average particle diameter of 1 to 15 μm are added by 5 to 30 mass % to the alloy powder disclosed in Patent Document 2 that has an average particle diameter of 15 to 40 μm, being composed of 55 to 85 mass % of Ag, 5 to 25 mass % of In, 0.2 to 2.0 mass % of Ti, and the balance Cu with inevitable impurities. The proposal is based on the finding that, by post-adding Ag powder particles having an appropriate particle diameter and particle size distribution by an appropriate amount to the alloy powder using an Ag—Cu—In—Ti-based brazing material as a base material, the scale-like irregularities which are formed on the surface of a brazing material layer in bonding of a copper plate are moderated, whereby the bonding strength can be improved.